In the future, Licensed-Assisted Access (LAA) will likely allow LTE equipment to operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum will be used as a complement to the licensed spectrum. Accordingly, devices may connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called listen-before-talk (LBT) method may be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
Due to the LBT procedure, the first slot in which the LAA SCell or LAA UE is permitted to transmit cannot be predicted in advance. This makes it difficult to pre-compute the data payload since several parameters are currently dependent on the slot number in which data is transmitted.
LTE uses OFDM (orthogonal frequency-division multiplexing) in the downlink and DFT-spread OFDM (also referred to as single-carrier FDMA) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid. FIG. 1 illustrates an example time-frequency grid 100 where each resource element 110 corresponds to one OFDM subcarrier 112 during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA (single carrier-frequency division multiple access) symbols in the time domain as OFDM symbols in the downlink.
FIG. 2 illustrates an exemplary time-domain structure 200. As depicted, LTE downlink transmissions are organized into radio frames 202 of 10 ms. As shown in FIG. 2, each radio frame consists of ten equally-sized subframes 204 of length Tsubframe=1 ms. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled. FIG. 3 depicts an exemplary downlink subframe structure 300. As shown, in each subframe 302, the base station transmits control information in a control region 304. The control information identifies to which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
LTE Rel-11 and later embodiments allow the above-described resource assignments to also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). Conversely, for Rel-8 to Rel-10 embodiments, only Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in the above FIG. 3 are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
The generation of the baseband transmit signal on the physical shared channels for either the DL or UL generally involve scrambling, modulation mapping, layer mapping, precoding, and RE mapping. FIG. 4 illustrates an exemplary specific baseband processing chain 400 for the UL PUSCH (Physical Uplink Shared Channel) that includes scramblers 402, modulation mappers 404, layer mapper 406, transform precoders 408, precoder 410, resource element mappers 412, and SC-FDMA signal generators 414. For PUSCH scrambling, the initialization of the scrambling sequence generator at the start of each subframe is a function of the current slot number ns. This is also true for PDSCH (Physical Downlink Shared Channel) scrambling on the DL.
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. FIG. 5 illustrates a carrier aggregation scheme 500, according to an exemplary embodiment. A CA-capable UE is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
In certain embodiments, the number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to a scenario wherein the number of CCs in downlink and uplink are the same. Conversely, an asymmetric configuration refers to a scenario wherein the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. For example, a terminal may support more downlink CCs than uplink CCs even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a key feature of carrier aggregation is the ability to perform cross-carrier scheduling. This mechanism allows a (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a wireless device expects to receive scheduling messages on the (E)PDCCH on just one CC. The CC may be the same CC or a different CC via cross-carrier scheduling. The mapping from (E)PDCCH to PDSCH may also be configured semi-statically.
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. FIG. 6 illustrates an example CSMA/CA scheme, according to certain embodiments. As depicted, the channel is sensed to perform a clear channel assessment (CCA) at first time interval 602. A transmission is initiated at a second time interval 604 only if the channel is declared as Idle at first time interval 602. In case the channel is declared as Busy during first time interval 602, the transmission is essentially deferred until the channel is deemed to be Idle. When the range of several APs using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded.
Conventionally, the spectrum used by LTE is dedicated to LTE. This has the advantage that an LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited and cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects the channel is occupied.
One way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. FIG. 7 illustrates an exemplary network system 700 providing LAA in the unlicensed spectrum using LTE carrier aggregation. As depicted, a wireless device 710 is connected to a PCell 720 in the licensed band and one or more SCells 730 in the unlicensed band. As described herein, a secondary cell in unlicensed spectrum may be referred to as a licensed-assisted access secondary cell (LAA SCell).
To operate in unlicensed bands, a wireless device 710 must obey certain rules. For example, a transmitting wireless device 710 should listen on the carrier before the transmitting wireless device 710 begins to transmit. If the medium is free, the transmitting wireless device 710 can transmit. Conversely, if the medium is busy, such as when another wireless device or node is transmitting, the transmitting wireless device 710 should suppress the transmission and try again at a later time. This process of listening before transmitting and only transmitting when the medium is free may be referred to as listen before talk (LBT). As described, LBT results in transmissions in the unlicensed band being delayed until the medium becomes free again. Where there is no coordination between the transmitting wireless devices and nodes (which is often the case), the delay may appear random.
In LTE, a hybrid automatic repeat request (HARQ) protocol is used for transmissions in both UL and DL. Per the HARQ protocol, the transmitting wireless device 710 will perform transmissions of data to the receiver until the receiver indicates that the data was successfully received or a maximum number of transmissions have been reached. The indication of a successful transmission may be in the form of a positive HARQ feedback message, which may also referred to as an acknowledgement or ACK. When the receiver receives one reception but fails to successfully decode the data, the receiver will request the transmitting wireless device 710 to retransmit the data. Such a request may include a negative HARQ feedback, which may also be referred to as a NACK.
As described above, the transmitting wireless device 710 may transmit the data several times before it is successfully decoded. In such a case, the receiver can combine all receptions to make accurate decoding more likely since the receiver is more likely to correctly decode data with a higher number of receptions.
In uplink the HARQ timing is synchronous. This means that the timing between the UL transmissions and the HARQ feedback in the DL as well as the timing between the HARQ feedback and retransmissions is fixed and in frequency division duplex (FDD) is fixed to be 4 ms, in some embodiments, which leads to an 8 ms HARQ round trip time.
When wireless device 710 receives a PDCCH with an UL grant for new transmission (e.g. with NDI=1 and RVI=0), wireless device 710 will perform the PUSCH transmission 4 ms later. If the data was not correctly decoded by the network, the network will respond with a NACK to prompt wireless device 710 to perform the retransmission with another redundancy version. Alternatively, the network may send another PDCCH grant for adaptive retransmission in case of DTX. Wireless device 710 will perform a retransmission 4 ms later. If the data is correctly decoded after the retransmission, the network will respond with an ACK and may send a PDCCH with an UL grant with toggled NDI to trigger wireless device 710 to send the next data.
FIG. 8 illustrates an example HARQ process. As depicted, wireless device 710 receives an initial grant 810 at subframe 0 and performs an initial transmission 812 in subframe 4. In the depicted example scenario, the initial transmission 812 is lost. As a result, wireless device 710 receives a NACK or PDCCH with an UL grant order 814 in subframe 8 and performs a re-transmission 816 in subframe 12. Because re-transmission 816 is successfully decoded by the receiving network node, wireless device 710 receives an ACK 818 in subframe 16.
When wireless device 710 receives an initial grant from the network node, certain components and elements of the wireless device 710 cooperate to generate a packet within the parameters of the initial grant. For example, the PHY layer of wireless device 710 may request the MAC to provide a MAC PDU of a size appropriate for the grant. The MAC layer may then, in turn, request the RLC layer to provide an RLC PDU which fits in the MAC PDU. More specifically, the RLC PDU size may be the MAC PDU size minus headers and MAC CEs. The RLC layer of wireless device 710 may then take data from the top of the buffer to construct the RLC PDU and provide it to the MAC layer. The MAC layer may use the data to construct the MAC PDU and then provide the MAC PDU to PHY. The PHY then performs the transmission. If wireless device 710 receives a second, subsequent UL grant valid for a following TTI, wireless device 710 will repeat the procedure and construct a new MAC PDU with data from the top of the buffer. Within the buffer, the top has moved since wireless device 710 removed data from the buffer to fulfill the first grant.
In certain LAA scenarios, wireless device 710 may also drop an initial transmission (or any subsequent transmission) in response to a LBT procedure that determines the channel is busy. For example, wireless device 710 may receive a grant in subframe n and prepare an UL transmission to be sent in subframe n+4. If the channel again happens to be busy in subframe n+4, wireless device 710 will drop the transmission. At the earliest, wireless device 710 can resend the data one HARQ RTT later.
FIG. 9 illustrates an exemplary LAA process wherein an initial transmission is dropped, in certain embodiments. Specifically, FIG. 9 depicts wireless device 710 receiving an initial grant 910 at subframe 0. Wireless device 710 then performs an LBT procedure, determines the channel is busy, and drops initial transmission 912 in subframe 4. Wireless device 710 then receive a subsequent PDCCH grant 914 and re-transmits the data in subframe 12. If the data is successfully received and decoded, wireless device 701 receives a HARQ ACK 918 in subframe 16.
In certain LAA scenarios, the data transmissions may not only delayed but also received in the wrong order. Consider, for example, the simplified example depicted in FIG. 10. Wireless device 710 receives a first grant 1010 for a first data transmission of 100 bytes in subframe 5 and a second grant 1012 for a second data transmission of 200 bytes in subframe 6. Due to the LBT process and the channel being busy during subframe 5, wireless device 710 may fail to transmit the first data transmission 1014 of 100 bytes in subframe 5 but succeed in transmitting the second data transmission 1016 of 200 bytes subframe 6. As a result, the receiving network node receives the second data transmission 1016 of 200 bytes from somewhere in the middle of the buffer 1020 before the network node receives the first data transmission 1014 of 100 bytes, which are actually from the top of the buffer 1020. The first data transmission 1014 would then be retransmitted in a third data transmission 1018 during subframe 13 or some time thereafter. Because wireless device 710 sends the data transmissions in the order, unnecessary delay and reduced user experience may result.